The presently disclosed embodiments are directed toward methods and systems related to rendering images with hypochromatic or relatively low chroma colorants in addition to corresponding conventional or relatively higher chroma colorants such as the conventional cyan, magenta, yellow and black colorants. Using hypochromatic colorants, such as light cyan and light magenta in addition to the conventional colorants allow images to be rendered with smoother gradations and reduced texture and visual noise than is possible with conventional colorants alone. Embodiments will be described that reduce colorant consumption and allow for clustered shape halftone screens such as clustered dot and line halftone screens to be used in halftoning more than four separations.
Image rendering technologies are associated with physical restrictions. For example, display devices are limited in the amount of phosphor or the number of light emitting elements that can be included in the given area and/or in the dynamic range of the amount of light that can be produced by such elements. One physical restriction experienced in printing systems is referred to as an ink limit. For example, many electrophotographic or xerographic rendering devices exhibit ink limits of about 240-280%. That is, the print media, such as paper, can typically accept 2.4-2.8 layers of ink or toner. Attempts to apply amounts of colorant beyond the ink limit result in image quality degradation due to retransfer and fusing considerations. This is an issue even in conventional printing systems where there are four colorants and theoretically, it might be desirable to apply three or four layers of colorant (i.e., 300% or 400% inking). This issue is exacerbated when additional colorants, such as a first, second or third hypocolorant (e.g., light cyan, light magenta, and/or grey) is added to the pallette of available inks or toners.
Another issue related to the use of additional colorants is that of halftone screen selection. Each additional halftone screen required to render an image increases the likelihood of the generation of objectionable moiré. Stochastic screens can be used to mitigate this, however, stochastic screens can lead to a noisy or grainy appearance that is inappropriate for the high quality applications typically associated with hypochromatic colorants. Accordingly, clustered shape halftone screens such as, clustered dot or clustered line screens are preferred. However, as indicated above, if clustered screens are not selected carefully, the screens selected for each color separation may interact with one another to create objectionable moiré patterns. While solutions to the moiré issue have been found for the conventional colorants (i.e., CMYK) efforts to find methods for halftoning 5, 6, 7 or more colorants are ongoing. For example, U.S. Pat. No. 5,892,891 to Dalal et al. discusses using the same screen for a hi-fi colorant and its complementary colorant (e.g., cyan and orange). Those techniques are not applicable to hypocolorants. In “Halftone-Angle Combinations for N Color Separations”, M. Coudray suggests using the same screen for a lightened colorant and a different conventional colorant (e.g., light magenta and conventional cyan). However, in at least some instances this suggested technique could lead to significant moiré and color shifts for small registration errors between color separations.
Accordingly, there is a desire for color management techniques that are applicable to hypocolorant environments that intelligently use the available ink budget or limit. Additionally, there is a need for halftoning methods that allow for the use of hypocolorants in combination with the conventional colorants while minimizing any aggravation of the moiré issue.